a compound, heat is evolved. The chief source of our artificial heat is combustion; and combustion is simply chemical union-the union of the oxygen of the atmosphere with the carbon and hydrogen of the fuel, producing carbonic acid and water. Chemical action is sometimes attended with vast visible energy, as when gunpowder explodes. 46. Potential Molecular Energy.-Before chemical affinity will act, the atoms must be brought into close proximity by being dissolved together or heated together. While two atoms are beyond the range of affinity, they are like the earth and the suspended stone; they manifest no actual energy, but they have potential energy, which becomes actual or kinetic when they are brought within range. We may thus look upon a heap of fuel as a store of potential energy, the necessary oxygen being always at hand, and only initial heat being necessary to bring the affinity into play. 47. Energy always the result of previous Work.— But a question arises here: The potential energy of the raised weight or the bent bow did not come uncaused; it required work—that is, expended kinetic energy, to lift the weight or to bend the bow, in order to put it in the position of advantage. Has the energy stored up in fuel cost any work? It has; and thus: Fuel consists mainly of vegetable products-wood and coal-which is simply fossil vegetable matter; and the most valuable ingredient of the fuel as a source of heat is carbon. But the chief food of plants is not carbon by itself, but carbonic acid, or carbon and oxygen combined. In this condition there is no energy possible. The two elements are in the relation of the stone and the earth when the stone is resting upon the earth; and they must be separated in order to restore their power. This work of separation is performed in the plant. A plant absorbs carbonic acid from the soil by its roots, and from the air by its leaves; and while this is circulating through the tissues, the union of the two elements is dissolved, the carbon is fixed in the solid substance of the plant, and the oxygen is exhaled or otherwise disposed of. This chemical analysis is effected through the energy of the sun's rays; for without the heat and light of the sun, there would be no vegetation. The stores of coal, then, laid up in the strata of the earth's crust are the work of the sun's rays countless ages ago-bottled sunbeams,' they have been called. The power of a waterfall to do work can be traced to the same source-the energy of the sun's rays; it is by it that water is raised high into the clouds, and thus gets energy of position. MAGNETISM AND ELECTRICITY. 48. How distinguished from the Forces already considered. The natural forces as yet considered are permanent in the bodies manifesting them. Electricity and Magnetism are only temporary; they may be imparted to a body, and again vanish. The phenomena of Electricity and Magnetism form now one of the most extensive departments of physical science; we have here to consider them only in connection with the doctrine of Energy. MAGNETISM. 49. Magnets Natural and Artificial.-A certain kind of iron-stone, or ore of iron, has long been known to have the power of attracting small bits of iron or steel. A bar of steel, if repeatedly rubbed with this loadstone, as it is called, acquires the same properties, and is called an artificial magnet; while the loadstone is a natural magnet. 50. Poles of the Magnet.-If over a bar-magnet we scatter iron-filings, they will be found to adhere in tufts to the ends, but none at the middle. The magnetic force is thus concentrated at or near the ends, which are therefore called the poles of the magnet, from a remarkable property they possess. If a bar-magnet is delicately settled in one position, If disturbed, it always same end always points balanced on a pivot, or suspended by the middle in a thread, it will not rest till it has which is nearly north and south. returns to that position, and the to the same pole of the earth. The end that points to the north is called the north pole of the magnet, and the other end the south pole. 51. Poles always in Pairs-Polarity.-We cannot have a magnet with only one pole. If a magnet is broken into two, each piece becomes a complete magnet, with two opposite poles, a north and a south. This exhibition of twin forces in opposite sides of a body is known as polarity. It is not peculiar to magnetism, but is seen also in all electrical phenomena, one kind of electricity being always accompanied by the opposite kind. C 52. Law of Polar Action.-Place a small bar-magnet afloat on a piece of cork in a basin of water; hold another magnet in the hand, and present its two poles successively to those of the floating magnet; the result of your experiments will be this: Like poles repel each other; unlike poles attract each other. 53. Magnetic Induction.--A bar of soft iron has of itself no power to attract another piece of iron; but in presence of a magnet it instantly assumes that power. n Fig. 9. Let a short bar of soft iron, ns (fig. 9), be suspended from the south pole of the magnet NS; it becomes for the time a If s is dipped magnet itself, its south pole being s into iron filings, the filings adhere to it; but if the bar is removed from NS, the filings fall off instantly. The same effect follows when ns is merely brought near to S without touching it. The temporary magnetism of ns is said to be induced by the presence of NS. If instead of soft iron, ns is of tempered steel, the induction does not take place at once, but only after repeated friction with the magnet; and then it retains the property even when removed. The great force that magnetic attraction may exert is best seen by making the magnet in the form of a horseshoe. But more powerful than any steel magnets are those of soft iron produced by the induction of the electric current (see par. 71). 54. The Mariner's Compass.-The directive property of magnetism furnishes that most useful instrument the Mariner's Compass. It consists of a light magnet or needle balanced on a pivot: by always pointing north and south, it shows the quarters of the heavens when there are no other marks. It is only in rare instances that its direction is exactly north and south; the deviation from the meridian is called the declination of the needle, and varies in different places, and also from age to age. At Greenwich, for instance, the needle in 1880 pointed 18° 32′ west of north; but it is coming nearer and nearer to the true north by, on an average, 8' yearly. 55. The Earth a great Magnet.-The theory of the directive property of magnetism is that the earth itself is a great magnet, having its opposite centres of force situated not exactly in, but near the geographical poles. The poles of this great magnet, then, attract the opposite poles of the magnetic needle, and make it lie in the direction of the line between them. The magnetism of the earth is believed to be affected by commotions on the sun's surface; it is often thrown into violent agitations, which cause the magnetic needles of observatories to vibrate from side to side. Magnetism thus forms a bond of sympathy between the members of the solar system. ELECTRICITY-FRICTIONAL. 56. Origin of the Name.-When a stick of sealingwax is rubbed with a piece of woollen cloth, it is found to attract pieces of straw or feathers, which, after touching it, seem to fly away from it. Many other substances may, in various ways, be made to manifest similar powers. To those phenomena the name of Electricity has been given, because amber, in Greek electron, was the first substance in which the property was observed. |